A large-scale analysis of mRNAs expressed by primary mesenchyme cells of the sea urchin embryo

The primary mesenchyme cells (PMCs) of the sea urchin embryo have been an i mportant model system for the analysis of cell behavior during gastrulation . To gain an improved understanding of the molecular basis of PMC behavior, a set of 8293 expressed sequenced tags (ESTs) was derived from an enriched population of mid-gastrula stage PMCs, These ESTs represented approximatel y 1200 distinct proteins, or about 15% of the mRNAs expressed by the gastru la stage embryo. 655 proteins were similar (P < 10(-7) by BLAST comparisons ) to other proteins in GenBank, For which some information is available con cerning expression and/or function. Another 116 were similar to ESTs identi fied in other organisms, but not further characterized. We conservatively e stimate that sequences encoding at least 435 additional proteins were inclu ded in the pool of ESTs that did not yield matches by BLAST analysis. The c ollection of newly identified proteins includes many candidate regulators o f primary mesenchyme morphogenesis, including PMC-specific extracellular ma trix proteins, cell surface proteins, spicule matrix proteins and transcrip tion factors. This work provides a basis for linking specific molecular cha nges to specific cell behaviors during gastrulation, Our analysis has also led to the cloning of several key components of signaling pathways that pla y crucial roles in early sea urchin development.